TECTONIC EVOLUTION OF THE SOUTHEASTERN END OF THE STE. GENEVIEVE FAULT SYSTEM, ILLINOIS: THE WOLF CREEK FAULT ZONE

The Ste. Genevieve Fault System is a prominent WNW-trending structure in the U. S. continental interior that forms the boundary between the Ozark Dome and Illinois Basin. We investigated the structural geometry and kinematics in detail at the southeastern termination of the SGFS by examining outcrops and drill holes in the Wolf Creek Fault Zone, a brittle fault array that is oblique to the main trend. While normal faulting, indicative of local crustal stretching, is dominant, the region also contains subtle structures indicative of local crustal shortening. New geologic mapping and shallow drilling in a 45 km² (18 mi²) region that includes the Wolf Creek Fault Zone indicate that normal faulting in the zone produced several tilted fault blocks of Mississippian strata. This tilting occurred prior to deposition of the sub-horizontal beds of the Pennsylvanian Caseyville Formation, which indicates that the deformation took place in the Late Mississippian/Early Pennsylvanian. We postulate that the deformation occurred due to two episodes of tectonic stress at a stepover at the southeastern end of the Ste. Genevieve Fault System. First, transpression associated with left-lateral movement produced shortening structures. Later, transtension associated with right-lateral movement produced an extensional pull-apart zone. Similar episodes of deformation have been found in other fault zones in the Illinois Basin. The timing of activity in the Wolf Creek Fault Zone implies that it records a period of Alleghanian/Ouachita continental-interior fault reactivation and propagation, and thus is a Midcontinent manifestation of the Ancestral Rockies event.